This invention relates to apparatus for electrocoating. The apparatus, according to this invention, is particularly adapted to the electrocoating of nestable objects such as can ends. More specifically, the invention is particularly adapted for "post repair" of ends.
In the formation of container ends for packaging beer or other carbonated beverages, it is necessary to have the internal surfaces, which are exposed to the product, protected from environmental product attack. In the formation of such container ends, a blank flat sheet material is normally coated by applying a base coat with a roller or other device onto both surfaces of the metal stock prior to the fabrication of the container end. Flat blanks are then cut from the stock and converted into container ends which include a peripheral flange that is utilized for double seaming the end to a container body. In addition, for beer and carbonated beverage products, the container end also has a tear strip formed therein with a tab attached to the tear strip.
During the conversion of the flat blank into the finished end, it is virtually impossible to prevent the removal of some of the coating from certain areas, such as the areas where the frangible connection is produced between the tear strip and the remainder of the end panel. This may result in bare metal spots or uncoated surfaces, on the end, particularly the internal surface that is ultimately exposed to the product. Thus, manufacturers normally apply a subsequent coating to at least the exposed bare metal areas or surfaces of the container end, which is commonly termed "post repair" of the ends.
For years it has been customary to spray a top coat on the interior surface of the fabricated end to seal any scratches, breaks or discontinuities that are produced in the coating during the conversion from the flat blank to the finished end. While this has been found to be an acceptable practice, there are certain shortcomings in utilizing spray coating. One of these shortcomings is the fact that the entire surface must be recoated to insure that all bare spots are covered, which results in wasting coating material in areas where no additional coating is necessary. Another shortcoming is that the spray coating usually incorporates solvents which are dangerous to inhale and must be removed by heating the end.
While the electrodeposition of resinous materials on metal surfaces has been known and proposed for can bodies (see for example, U.S. Pat. Nos. 3,647,675; 3,694,336 and 3,801,485), to the best of applicant's knowledge a successful commercial process for coating can ends for use with beer and carbonated beverages has heretofore not been developed.
There are several problems inherent in any attempt to electrocoat such ends and particularly the "post repair" of coated ends. For example, in order to completely coat the ends, the electrolytic coating solution must be in contact with all surface areas to be coated. Therefore, a means must be devised to move the ends through the coating area which will allow for such full contact. Furthermore, substantial build-up of electrocoated resin on the metallic surfaces making electrical contact with the end should be avoided; otherwise, electrocoating must be interrupted periodically while such surfaces are cleaned. Another problem is encountered when can ends having an easy open end feature are to be electrocoated. These ends generally feature pull tabs that are held in place by a rivet integrally formed in the end, the exposed end of which is located in a recessed area in the bottom surface of the end. Achieving a uniform coating of a rivet-containing end with resin, therefore, has proven to be especially difficult.
A further problem in "post repair" coating of ends is the difficulty in producing metal-to-metal contact between the coated end and an energy source which is necessary for proper electrocoating.